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Faster-than-light observations and experiments
There are situations in which it may seem that matter, energy, or information travels at speeds greater than c , but they do not. For example, as is discussed in the propagation of light in a medium section below, many wave velocities can exceed c . For example, the phase velocity of X-rays through most glasses can routinely exceed c , but phase velocity does not determine the velocity at which waves convey information. If a laser beam is swept quickly across a distant object, the spot of light can move faster than c , although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed c . However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than c , after a delay in time. In neither case does any matter, energy, or informat...
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